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Update: Influenza Activity -- United States, 1996-97 Season

Influenza activity in the United States increased from mid-November through December 1996, peaked during December 22, 1996-January 4, 1997, and began to decline during January 5-18. Laboratory-confirmed influenza type A and/or type B has been reported in all states this season. This report summarizes influenza surveillance data from September 29, 1996, through January 18, 1997.

As of January 18, World Health Organization collaborating laboratories in the United States had tested 20,754 specimens for respiratory viruses, and 4063 (20%) were positive for influenza. Of these isolates, 3959 (97%) were influenza type A, and 104 (3%) were influenza type B. The number of influenza isolates and percentage of specimens positive for influenza increased steadily during November and December. During the weeks ending December 28 and January 4, 31% of specimens tested were positive for influenza; however, the percentage positive for influenza declined to 25% during the week ending January 11 and to 18% during the week ending January 18. The number of influenza type B isolates increased each week during December and accounted for 68 (5%) of 1376 isolates reported during December 29-January 18. Although influenza type B viruses have been isolated in all nine regions of the United States, 51 (49%) of the 104 influenza type B isolates reported this season were from the Pacific region.

State and territorial epidemiologists first reported regional influenza activity * for the week ending October 19, 1996. During each of the subsequent 4 weeks, one or two states reported regional activity. Widespread activity was first reported during the week ending November 23 (week 47) from two states (Colorado and Pennsylvania), and the number of states reporting regional activity during week 47 increased to seven. The number of states reporting regional or widespread activity increased during December and peaked at 38 during the week ending January 4, then declined to 31 during the week ending January 18. Most laboratory-confirmed influenza outbreaks reported by states to CDC have occurred among elderly nursing-home residents, although some reported outbreaks have been among children and young adults.

During the week ending December 7, the percentage of visits to U.S. sentinel physicians for influenza-like illness (ILI) increased above baseline levels (0-3%) for the first time this season and ranged from 5% to 7% through the week ending January 4. The percentage of patient visits for ILI returned to baseline levels (3%) during the week ending January 11 and remained at baseline levels (2%) during the week ending January 18. The percentage of deaths attributed to pneumonia and influenza (P&I) exceeded the epidemic threshold ** for the first time this season during the week ending December 14 and has continued to increase each week through the week ending January 18. During the week ending January 18, 8.6% of deaths were attributed to P&I, which is above the epidemic threshold of 7.2% for the week.

The trivalent vaccine prepared for the 1996-97 influenza season contains A/Texas/ 36/91-like (H1N1), A/Wuhan/359/95-like (H3N2), and B/Beijing/184/93-like viral antigens. U.S. manufacturers used A/Nanchang/933/95(H3N2) and B/Harbin/07/94 viruses for the A/Wuhan/359/95-like and B/Beijing/184/93-like antigens, respectively, because of their growth properties. All 105 influenza A(H3N2) isolates antigenically characterized by CDC were closely related to A/Wuhan/359/95 and A/Nanchang/933/95 (Table_1). The five antigenically characterized influenza B isolates were B/Beijing/184/93-like.

Reported by: Participating state and territorial epidemiologists and state public health laboratory directors. World Health Organization collaborating laboratories. Sentinel Physicians Influenza Surveillance System. Influenza Br, and WHO Collaborating Center for Surveillance, Epidemiology, and Control of Influenza, Div of Viral and Rickettsial Diseases, National Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: The pattern of influenza activity during mid-November 1996-January 18, 1997, has been consistent with previous influenza seasons dominated by influenza A(H3N2) viruses. Since the emergence of influenza A(H3N2) viruses in 1968, seasons during which these viruses predominate have been associated with higher morbidity and mortality, particularly among the elderly, than have seasons during which influenza A(H1N1) or influenza type B viruses predominated (1,2). When influenza strains included in vaccines are closely matched with circulating strains, influenza vaccine is approximately 70% effective in preventing ILI in healthy adults aged less than 65 years. Because of decreased immunologic response among persons aged greater than or equal to 65 years, influenza vaccine may be only 30%-40% effective in preventing ILI among nursing-home residents; however, among such groups, influenza vaccine may be 50%-60% effective in preventing pneumonia and hospitalization and 80% effective in preventing death (3).

Despite the close match between influenza vaccine strains and strains circulating during the current season, the potential remains for influenza outbreaks among vaccinated groups. Nursing homes and other facilities providing care for persons at risk for influenza-associated complications should consider using the antiviral agents amantadine hydrochloride or rimantadine hydrochloride for prophylaxis and/or treatment during institutional outbreaks of influenza type A (3).

The increase in the proportion of influenza type B viruses in recent weeks emphasizes the importance of continued surveillance to detect changes in the relative proportions of circulating influenza virus types or subtypes. Influenza virus types or subtypes that were not predominant during the early part of the season often increase in number toward the end of the season. Current surveillance data indicate circulation of both influenza type A and influenza type B viruses. Because amantadine and rimantadine are effective only against influenza type A viruses, the use of rapid diagnostic testing for influenza type A can be useful in guiding decisions regarding management of cases of influenza and responses to outbreaks.

Influenza surveillance data are updated weekly and are available through the CDC voice information system, telephone (404) 332-4551, or the fax information system, telephone (404) 332-4565, by requesting document no. 361100.


  1. Lui KL, Kendal AP. Impact of influenza epidemics on mortality in the United States from October 1972 to May 1985. Am J Public Health 1987;77:712-6.

  2. Noble GR. Epidemiological and clinical aspects of influenza. In: Beare AS, ed. Basic and applied research. Boca Raton, Florida: CRC Press, 1982:11-50.

  3. ACIP. Prevention and control of influenza: recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR 1996;45(no. RR-5).

* Levels of activity are 1) no activity; 2) sporadic -- sporadically occurring influenza-like illness (ILI) or culture-confirmed influenza, with no outbreaks detected; 3) regional -- outbreaks of ILI or culture-confirmed influenza in counties with a combined population of less than 50% of the state's total population; and 4) widespread -- outbreaks of ILI or culture-confirmed influenza in counties with a combined population of greater than or equal to 50% of the state's total population. 

** The epidemic threshold is 1.645 standard deviations above the seasonal baseline. The expected seasonal baseline is projected using a robust regression procedure in which a periodic regression model is applied to observed percentages of deaths from P&I since 1983.

Note: To print large tables and graphs users may have to change their printer settings to landscape and use a small font size.

TABLE 1. Hemagglutination-inhibition titers of influenza type A(H3N2) viruses with serum
specimens from infected ferrets *
                                                  Ferret antiserum
Viral antigen                 A/Johannesburg/33/94     A/Wuhan/359/95      A/Nanchang/933/95
Reference antigens
 A/Johannesburg/33/94          640                        40                  40
 A/Wuhan/359/95                 80                      1280                1280
 A/Nanchang/933/95              80                      1280                1280

Recent isolates
 A/New York/37/96              160                      1280                1280
 A/Minnesota/01/96              80                       640                1280
 A/Texas/09/96                  80                       640                1280
 A/Indiana/02/96               160                      1280                1280
 A/Washington/07/96             40                       640                1280
 A/California/11/96             80                      1280                1280
* A fourfold difference in hemagglutination-inhibition titers between two viruses is usually
  indicative of antigenic variation between viruses.

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